SEISMIC ANISOTROPY OF THE PAPUA NEW GUINEA (ULTRA)HIGH-PRESSURE TERRANE FROM IN SITU VELOCITY MEASUREMENTS AND EBSD MEASUREMENTS

The (ultra)high pressure terrane in the D’Entrecasteaux Islands of Papua New Guinea is the youngest known (Baldwin et al., 2008) and is composed of continental crust buried during arc-continent collision and exhumed during subsequent and currently active extension. Seismic velocities and seismic anisotropy of this region may shed light on the mechanisms that result in the exhumation of (ultra)high pressure rocks. We measured the velocities of these rocks directly using the pulse-transmission technique (e.g. Christensen, 1965), and we used electron backscatter diffraction (EBSD) to measure the crystallographic preferred orientations (CPO) of the constituent minerals and calculated their seismic properties using elastic constants (C_ij) and the method of Mainprice (1990). The CPOs for both quartz and plagioclase are weak in all samples; hornblende, clinozoisite, and clinopyroxene have relatively strong CPOs compatible with slip on [001](100) in hornblende, [010]{h01} in clinozoisite, and [001](010) in clinopyroxene; biotite and muscovite are minor phases (<10 vol%), and have CPOs that imply [uvw](001) slip. The calculated Vp and Vs1 range from 6.4–7.9, and 3.7–4.6 km/s, and the Vp/Vs ratios range from 1.67-1.77. The calculated anisotropies range from 2–8%, with the highest anisotropies in samples containing significant hornblende and clinozoisite. The calculated results differ from the laboratory measurements by <6% for Vp, 7% for Vs1 and Vs2, and 3% for Vp/Vs. The accuracy of the velocity calculations is limited by the availability of suitable C_ij, the spatial coverage of the EBSD maps and the number of data, and as such, further improvements can be expected.